1. Technical Field
The present invention relates to hermetically sealed packages and, more particularly, to dewars containing infrared detectors.
2. Discussion
Infrared detection systems are often used in conjunction with munition and night vision systems for sensing electromagnetic radiation in the wavelength range of 1 to 15 micrometers. Most infrared detection systems employ sensors, such as mercury-cadmium-telluride detectors, which are most sensitive when operating at approximately 77K. Therefore, a cryogenic cooling system is required to produce and maintain the required low operating temperatures. Typically, such cooling systems either take the form of a cryostat operating using the Joule Thompson effect or a Stirling cycle cryoengine.
The cooling system is used in conjunction with an evacuated dewar in which the detector is placed. Evacuation of the dewar is required to remove thermally conductive gases which would otherwise occupy the vacuum space surrounding the detector so that potential heat loss through convection and conduction during cryogenic operation is minimized. The evacuated dewar also inhibits moisture from condensing on the detector. The infrared detector is typically cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("coldfinger") of the cryogenic cooling system. The coldfinger is a tubular member having an end ("cold end") which is cooled and which supports the detector and its related readout components. During operation, the cryogenic cooling system expands a fluid, such as helium, in the coldfinger near the coldend which, in turn, absorbs thermal energy for cooling the detector.
While traditional evacuated dewars have generally operated satisfactorily, they do have some design constraints which detrimentally effect their manufacturability and reliability. For example, the choice of materials that can be used to fabricate the various components of the dewar are somewhat limited and expensive since it is necessary to choose materials having special characteristics such as low diffusivity, low out-gassing and other properties. Furthermore, implementing the necessary closure techniques (i.e., hermetic welding and brazing) required to create the vacuum inside the dewar is often costly and unreliable in that it is difficult to ensure that vacuum integrity is maintained over a long period of time. It is known that a prime cause of detector failure is the gradual degradation of vacuum integrity in the dewar due to internal out-gassing from the various components surrounding the detector which are also exposed to the vacuum environment. Such vacuum degradation eventually leads to a condition in which the cooling system is no longer able to effectively cool the detector to the desired temperature within the desired time period for sensitive detection of incoming infrared radiation. Thus, a primary design criteria for most infrared detection systems, is an extended "shelf-life" of the vacuum within the dewar assembly.
In order to reduce internal out-gassing, it is known to provide at least one getter in the vacuum space for gettering gas molecules therefrom. Conventionally, most infrared detector assemblies use electrically activated or "wire-heated" getters which require vacuum feedthrough pins protruding through the dewar housing into the vacuum space surrounding the detector. Numerous manufacturing operations are required to assemble and hermetically seal the wire-heated getters in conventional detector assemblies. Thereafter, the wire-heated getters are activated by being taken to a very high temperature in the range of about 900.degree. to 1000.degree. C. following evacuation and hermetic closure processing of the detector assembly. As such, costly internal shielding is required to protect the sensitive detector components, welds and brazed joints from the extremely high getter activation temperatures associated with wire-heated getters. Similarly, since the wire-heated getters require specific placement in the dewar, fewer getters can be used which results in less gas capacity and, in turn, a shorter vacuum life. Furthermore, an increased risk of vacuum failure is associated with dewars having wire-heated getters due to loss of hermeticity at the getter feedthroughs. These drawbacks result in less dewar reliability and higher manufacturing and scrap costs associated with fabrication of conventional "wire-heated" infrared dewar assemblies. In addition, wire-heated getters are susceptible to failure due to environmental stresses acting on the wire connections which can break or be electrically shorted during operation.